Makefile: use FUZZ_CXXFLAGS for linking fuzzers

OSS-Fuzz requires C++-specific flags to link fuzzers. Passing these in
CFLAGS causes lots of build warnings. Using separate FUZZ_CXXFLAGS
avoids this.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -3083,14 +3083,16 @@ cover_db_html: cover_db
 # An example command to build against libFuzzer from LLVM 4.0.0:
 #
 # make CC=clang CXX=clang++ \
-#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
+#      FUZZ_CXXFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
 #      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
 #      fuzz-all
 #
+FUZZ_CXXFLAGS ?= $(CFLAGS)
+
 .PHONY: fuzz-all
 $(FUZZ_PROGRAMS): all
-	$(QUIET_LINK)$(CXX) $(CFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
+	$(QUIET_LINK)$(CXX) $(FUZZ_CXXFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
 		$(XDIFF_OBJS) $(EXTLIBS) git.o $@.o $(LIB_FUZZING_ENGINE) -o $@
 fuzz-all: $(FUZZ_PROGRAMS)